This invention relates to a packaging material for photosensitive materials, such as photographic material, food stuffs, etc., and more particularly to a packaging material for photosensitive materials which has high physical strength, can be manufactured at low cost, and from which an easy opening bag can be made.
Photosensitive materials such as photographic film or paper are vulnerable to light, so they must be wrapped with a packaging material that provides a complete shield from light. If the packaging material is used as a bag, it must have sufficient physical strength (i.e., tensile strength and tear strength) for the specific size and weight of the material to be packed. The opening of the bag containing the photosensitive material is heat-sealed to make the bag completely lightproof and moisture-proof. For this purpose, the packaging material is required to have good heat-sealability and heat seal strength. It is also required that the heat sealed bag be easily torn open and provide a straight tear line.
The configuration of the bag for packaging photosensitive materials can vary to some extent depending upon the use, but it is in most cases a bag having sides which are heat sealed completely. A known structure of a packaging material for making such a bag is schematically represented in accompanying FIG. 1, wherein a lightproof low density polyethylene film 1 having carbon black or pigment incorporated therein is laminated with a flexible layer 2, such as of paper, aluminium foil, or cellophane by means of an adhesive layer 3. However, such conventional packaging material comprising undrawn low density polyethylene film 1 does not necessarily have high physical strength and moisture resistance, and to provide these properties, the thickness of the film 1 must be increased. Furthermore, the lightproof material such as carbon black or pigment incorporated in the film 1 has a tendency to decrease the physical strength of the film, and so, to provide an acceptable lightproof packaging material it is necessary to further increase the thickness of the film 1. However, making a bag of the thicker packaging material has the following disadvantages:
(1) the bag is difficult to tear open by hand, and even if a notch is provided to facilitate opening, the bag may tear in an unpredictable direction; PA1 (2) the increase in the film thickness results in a corresponding increase in the manufacturing cost of the packaging material; and PA1 (3) a lamination made of a thick film and paper or the like is easily affected by shrinkage of the film, and the resultant curling makes it difficult to make a bag of such packaging material.
A uniaxially-oriented film has extremely small tear strength in the direction of the axis of orientation and can be torn rectilineally, but taking it the other way round, this oriented film is very easy to tear when an article having protuberances is packed therein, and a hole or a tear are often made. In order to avoid these disadvantages, packaging materials comprising at least two uniaxially oriented films, which are bonded with an adhesive layer so that oriented axes of the films cross each other at an angle of 45.degree. to 90.degree. have conventionally been used. However, the materials as described above have such disadvantages that the increase in the film thickness results in a corresponding increase in the manufacturing cost of the packaging material, and the bag may tear in an unpredictable direction to give a notched opening part.